The high performance and low cost of modern desktop computers may be attributed to high device densities and high clock rates that are achievable within integrated circuits (ICs). The ICs used in desktop computers require a large number of input/output (I/O) interconnections and are operated at high clock rates, requiring short conductor paths between neighboring ICs. Flip-chip multichip modules, also referred to as MCMs, provide the I/O interconnections and short conductor paths. In the MCM, multiple unpackaged ICs or chips are mounted with the device side of each chip facing a common substrate. The chips are attached to the substrate using solder bumps while conductor paths between chips are provided by the substrate. The backside of each chip provides a thermal interface to a heat sink, used to cool the chips.
The high device densities and high clock rates of the chips produce corresponding increases in power density and power consumption. For example, a single chip within a MCM may be only 300 square millimeters in area but may dissipate as many as 60 Watts. As the performance trend in desktop computers is to drive integrated circuits toward even higher device densities and higher clock rates, the power density and power consumption of chips are also expected to increase. Thus, low thermal resistance at the thermal interface between the backside of the chips and the heat sink is essential to cool the chips and insure reliable operation of MCMs. Unfortunately, variations in substrate flatness, dimensional tolerance stack-up and mismatched thermal expansion coefficients of the chips, substrate and the heat sink have made it difficult to cool the chips using simple and cost-effective techniques.
A prior art chip cooling technique is described by Darveaux and Turlik, "Backside Cooling of Flip Chip Devices in Multichip Modules", ICMCM Proceedings, 1992, pp. 230-241. This technique is used in a Thermal Conduction Module (TCM) and incorporates a water-cooled heat sink with spring-loaded copper pistons that contact the backside of each chip within the TCM. The pistons provide mechanical compliance to accommodate dimensional tolerance stack-up and mismatches in thermal coefficients expansion between the heat sink, chips and substrate. Although this TCM may be feasible for cooling chips in mainframe computers, this approach is too expensive to be incorporated into low-cost, desktop computers.
Another prior art chip cooling technique is taught by Patel et al in U.S. Pat. No. 5,430,611 which issued Jul. 4, 1995. This technique uses a heat sink encapsulating a flip-chip multichip module (MCM). The backsides of the chips within the MCM are biased against the heat sink by a spring mechanism located on the side of a substrate opposite the side upon which the chips are mounted. Although this technique effectively cools the chips, the presence of the spring mechanism makes it difficult to bring electrical interconnections through the backside of the substrate and into the MCM using common electrical connectors, such as Pin Grid Arrays (PGAs).
In accordance with the illustrated preferred embodiment of the present invention a positional referencing scheme provides a low-cost thermal interface that has low thermal resistance between chips within a flip-chip multichip module (MCM) and a heat sink. The referencing scheme comprises a heat sink having a support ring that fits into a trough formed within a substrate of the MCM. The depth at which the support ring enters the trough references the position of the heat sink to the backside of the chips, cooling the chips and insuring reliable operation of the MCM. The trough is filled with a curable adhesive and when the adhesive cures, the heat sink is anchored to the substrate.